Q: A 10 kg box is at rest on a horizontal surface. The coefficient of static friction between the box and the surface is 0.4. What is the minimum horizontal force required to start moving the box?

$$F = \mu_s F_N = 0.4 \times 10 \times 9.8 = 39.2\ \mathrm{N}$$

Q: A 5 kg block is pulled across a horizontal surface with a constant velocity. If the coefficient of kinetic friction is 0.3, what is the magnitude of the frictional force?

$$F_{fr} = \mu_k F_N = 0.3 \times 5 \times 9.8 = 14.7\ \mathrm{N}$$

Q: A 2 kg object is suspended by a rope. What is the tension in the rope if the object is at rest?

$$T = mg = 2 \times 9.8 = 19.6\ \mathrm{N}$$

Q: A 15 kg crate is pulled with a force of 60 N at an angle of 30° above the horizontal. The coefficient of kinetic friction is 0.2. What is the normal force acting on the crate?

$$F_N = mg - F \sin 30^\circ = 15 \times 9.8 - 60 \times 0.5 = 147 - 30 = 117\ \mathrm{N}$$

Q: A 20 kg box is at rest on a horizontal surface. The coefficient of static friction is 0.5. If a horizontal force of 80 N is applied, will the box move?

No, because $$F_{applied} \u003c \mu_s F_N = 0.5 \times 20 \times 9.8 = 98\ \mathrm{N}$$

Q: A 50 kg skier slides down a slope inclined at 30° to the horizontal. The coefficient of kinetic friction is 0.1. What is the net force acting on the skier?

$$F_{net} = mg \sin 30^\circ - \mu_k mg \cos 30^\circ = 50 \times 9.8 \times 0.5 - 0.1 \times 50 \times 9.8 \times 0.866 = 245 - 42.43 = 202.57\ \mathrm{N}$$

Question 1

A 10 kg box is at rest on a horizontal surface. The coefficient of static friction between the box and the surface is 0.4. What is the minimum horizontal force required to start moving the box?

Accepted Answer

$$F = \mu_s F_N = 0.4 	imes 10 	imes 9.8 = 39.2\ \mathrm{N}$$

Question 2

Which of the following statements best describes Newton’s First Law of Motion?

Accepted Answer

An object will remain at rest or in uniform motion in a straight line unless acted upon by a net external force.

Question 3

A 70 kg astronaut weighs less on the Moon than on Earth. Which of the following quantities is the same on both the Moon and Earth?

Accepted Answer

The astronaut’s mass

Question 4

A car of mass $$m$$ accelerates from rest under a net force $$F. $$According to Newton’s Second Law, what is the acceleration $$a of $$the car?

Accepted Answer

$$a = \frac{F}{m}$$

Question 5

A person pushes a wall with a force of 50 N. According to Newton’s Third Law, what is the force exerted by the wall on the person?

Accepted Answer

50 N in the opposite direction

Question 6

A 5 kg block is pulled across a horizontal surface with a constant velocity. If the coefficient of kinetic friction is 0.3, what is the magnitude of the frictional force?

Accepted Answer

$$F_{fr} = \mu_k F_N = 0.3 	imes 5 	imes 9.8 = 14.7\ \mathrm{N}$$

Question 7

A 2 kg object is suspended by a rope. What is the tension in the rope if the object is at rest?

Accepted Answer

$$T = mg = 2 	imes 9.8 = 19.6\ \mathrm{N}$$

Question 8

A box is sliding down a frictionless incline at an angle $$	heta to $$the horizontal. Which component of gravity causes the box to accelerate down the incline?

Accepted Answer

$$mg \sin 	heta$$

Question 9

A 15 kg crate is pulled with a force of 60 N at an angle of 30° above the horizontal. The coefficient of kinetic friction is 0.2. What is the normal force acting on the crate?

Accepted Answer

$$F_N = mg - F \sin 30^\circ = 15 	imes 9.8 - 60 	imes 0.5 = 147 - 30 = 117\ \mathrm{N}$$

Question 10

A rocket launches vertically. According to Newton’s Third Law, what is the reaction force to the rocket’s exhaust gases being expelled downward?

Accepted Answer

The upward force propelling the rocket

Question 11

A 20 kg box is at rest on a horizontal surface. The coefficient of static friction is 0.5. If a horizontal force of 80 N is applied, will the box move?

Accepted Answer

No, because $$F_{applied} \u003c \mu_s F_N = 0.5 	imes 20 	imes 9.8 = 98\ \mathrm{N}$$

Question 12

Which of the following is NOT an inertial reference frame?

Accepted Answer

A car turning around a curve

Question 13

A 12 kg box and a 10 kg box are connected by a light rope and pulled with a force of 40 N on a frictionless surface. What is the acceleration of the system?

Accepted Answer

$$a = \frac{F}{m_{total}} = \frac{40}{12+10} = \frac{40}{22} = 1.82\ \mathrm{m/s^2$}$$

Question 14

A book rests on a table. Which pair of forces forms a Newton’s Third Law action-reaction pair?

Accepted Answer

The gravitational force on the book by Earth and the gravitational force on Earth by the book

Question 15

A 50 kg skier slides down a slope inclined at 30° to the horizontal. The coefficient of kinetic friction is 0.1. What is the net force acting on the skier?

Accepted Answer

$$F_{net} = mg \sin 30^\circ - \mu_k mg \cos 30^\circ = 50 	imes 9.8 	imes 0.5 - 0.1 	imes 50 	imes 9.8 	imes 0.866 = 245 - 42.43 = 202.57\ \mathrm{N}$$

Question 16

Which of the following best describes the normal force?

Accepted Answer

A force exerted perpendicular to a surface

Question 17

A 1 kg object is acted on by two forces: $$F_1 = 3$$\ \mathrm{N}$$ east and F_2$ = 4\ \mathrm{N}$$ north. What is the magnitude of the acceleration?

Accepted Answer

$$a = \frac{\sqrt{3^2$ + 4^2$}}{1} = 5\ \mathrm{m/s^2$}$$

Dynamics: Newton’s Laws of Motion – Study Notes

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